For Loop in C Programming – Syntax, Flowchart and Examples

Introduction to For Loop in C

The for loop in C programming is used to execute a block of code repeatedly for a known number of times until a specified condition is met. It is a fundamental control structure commonly used for iteration in C programs.

A for loop is especially useful when you need to repeat a task a specific number of times. It helps in writing efficient, clean, and organized code.

Common Use Cases of For Loop

• Generating number series — Even numbers, Fibonacci series, etc.
• Traversing arrays — Accessing elements in arrays, linked lists, or other data structures
• Processing student records — Looping through records to calculate grades
• Game development — Implementing repetitive tasks in games
• Searching — Searching for a value in an array
• Sorting — Sorting, searching, and traversing array elements
• CAPTCHA generation — Generating CAPTCHA codes for user verification
• Password validation — Validating password attempts within limited tries
• E-commerce — Calculating the total number of products and price in a shopping cart

For Loop Syntax in C

for (initialization; test_condition; update_expression) {
    /* statements or body of the loop */
}

Let's break down each part:

Infinite For Loop Syntax

If the initialization, test condition, and update expression are all omitted, the loop becomes an infinite loop.

for ( ; ; ) {
    /* statements or body of the loop */
}

For Loop Flowchart

The diagram below shows the step-by-step control flow of a for loop in C programming.

Control Flow of a For Loop in C:

1. Initialization (Declaration): This step executes only once, before the loop starts. In this section, the loop control variable is declared and initialized (e.g., int i = 0;). These variables have local scope within the loop.
2. Test Expression (Condition Check): The test condition is evaluated. If it returns true, the loop continues to the next step. If it returns false, the loop terminates, and control moves to the statement following the loop.
3. Loop Body Execution: If the test condition is satisfied, the statements inside the loop body are executed.
4. Update Expression: After the loop body executes, the control variable is updated (e.g., i++;). This step is crucial for progressing toward the loop termination condition.
5. Repeat: Control returns to step 2 (test expression). This cycle continues until the test condition evaluates to false.

C Program to Demonstrate Standard For Loop

The program below prints numbers from 1 to 10 using a standard for loop.

/* This C program prints the numbers from 1 to 10 using a for loop */

#include <stdio.h>

int main() {
    int i;
    
    printf("\nFor loop to display numbers from 1 to 10\n");
    
    for(i = 1; i <= 10; i++) {
        printf(" %d", i);
    }
    
    return 0;
}

Sample Output:

For loop to display numbers from 1 to 10
1 2 3 4 5 6 7 8 9 10

Explanation:

1. The variable i is declared as an integer.
2. The program first prints the message: For loop to display numbers from 1 to 10.
3. The for loop begins:
   • The loop starts by setting i = 1.
   • It checks if i <= 10. Since 1 is less than or equal to 10, the loop body executes, printing the current value of i.
   • After printing, i increments by 1 (i++).
   • The condition is checked again with the updated value of i.
   • This process repeats, printing numbers from 1 through 10.
   • When i becomes 11, the condition i <= 10 fails, and the loop stops.
4. The program then ends, having displayed the numbers 1 to 10 on the screen.

C Program to Illustrate Infinite For Loop (for;;)

The program below displays numbers from 1 to 10 using a for(;;) infinite loop with a break statement.

/* C program to display numbers from 1 to 10 using for(;;) loop */

#include <stdio.h>

int main() {
    int i = 1;
    
    for(;;) {
        printf(" %d", i);
        i++;
        
        if(i > 10)
            break;
    }
    
    return 0;
}

Sample Output:

1 2 3 4 5 6 7 8 9 10

Explanation:

1. We first declare and initialize i = 1.
2. The control enters the for(;;) loop. Since there is no initialization, condition, or update inside the loop header, the loop doesn't check any condition and directly enters the body.
3. The statement printf(" %d", i); is executed, which prints the value of i — in the first iteration, it's 1.
4. Then, i++; increments the value of i by 1. Now, i becomes 2.
5. The if(i > 10) condition is checked. Since i = 2, the condition is false, so the break statement is skipped.
6. Control goes back to the top of the loop. Again, since the loop header doesn't have any test condition, it continues and enters the loop body again.
7. This process repeats: i is printed, incremented, and then checked against the condition if(i > 10).
8. When i becomes 11, the condition if(i > 10) becomes true. So, the break statement executes, which stops the loop.
9. The control comes out of the loop, and since there are no more statements in the program, it ends.

#include <stdio.h>

int main() {
    int i, sum = 0;
    
    for(i = 2; i <= 100; i += 2) {
        sum = sum + i;
    }
    
    printf("Sum of even numbers from 1 to 100: %d\n", sum);
    return 0;
}

/* Output: Sum of even numbers from 1 to 100: 2550 */

Frequently Asked Questions About For Loop in C

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